1. Field of Invention
The present invention relates to a gas sensor based on a pH electrode which is used in the fields of medical care and chemical engineering.
2. Related Art
Any one of the conventional gas sensor in which the pH electrode (including an ion sensitive field effect transistor (ISFET)) is its fundamental, has the inner solution suitable for a gas to be measured, the pH electrode, and a reference electrode immersed in the inner solution, and these are covered by a gas permeable membrane. The composition of the inner solution is selected so that, when the gas to be measured is dissolved in the inner solution, the pH is changed.
For example, when the gas to be measured is carbon dioxide or ammonia, H+ ion or OHxe2x88x92 ion is generated by the following equilibrium reaction, causing the pH change of the solution. Herein, HCO3xe2x88x92 or NH4+ ion which is an ionic species generated when a gas molecule is dissolved in the water, is respectively called a conjugate ion to the carbon dioxide or ammonia. Normally, as the inner solution of the gas sensor based on the pH electrode, a water solution including the excessive conjugate ions to the gas to be measured is used.
CO2+H2Oxe2x86x92HCO3xe2x88x92+H+xe2x80x83xe2x80x83(1)
NH3+H2Oxe2x86x92NH4++OHxe2x88x92xe2x80x83xe2x80x83(2)
Further, for the purpose that the pH electrode is operated, the reference electrode is necessary, and in many cases, as the reference electrode, a silver wire (Ag/AgCl) whose surface is chlorinated, is used. Because the potential of this electrode is determined by the concentration of the chloride ion in the solution, normally, in the inner solution, the chloride ion of a predetermined concentration is included other than the conjugate ion.
The gas sensor of the above-described principle is generally called Severinghaus type gas sensor. The measurement result of the severinghaus type gas sensor is displayed normally as the gas partial pressure. When the conjugate ion excessively exists in the inner solution, the following relationship exists between the partial pressure of the gas to be measured and the pH of the inner solution. In the case of acidic gas,
pH=Axe2x88x92log Paxe2x80x83xe2x80x83(3),
in the case of basic gas,
pH=B+log Pbxe2x80x83xe2x80x83(4)
Herein, Pa and Pb are respectively the partial pressure of acidic gas and basic gas, and A and B are constants. Hereinafter, the relational expression between the partial pressure of the gas and the output voltage of the pH electrode in the case of the acidic gas will be deriverd.
Generally, between the output voltage V of the pH electrode and the pH, the expression (5) is generally established.
V=V0+S(pHxe2x88x92pH0)xe2x80x83xe2x80x83(5)
Herein, S is the pH sensitivity of the pH electrode, and pH0 and V0 are the pH of the inner solution and the output voltage of the pH electrode when the partial pressure of the gas is Pa0. Further, pH and V are the pH of the inner solution and the output voltage when the partial pressure of the gas is Pa. From the expressions (3) and (5), it is clear that the following relationship is derived between the pH electrode and the partial pressure of the gas.
V=V0xe2x88x92S(log Paxe2x88x92log Pa0)xe2x80x83xe2x80x83(6)
From this,
log Pa=log Pa0xe2x88x92(Vxe2x88x92V0)/Sxe2x80x83xe2x80x83(7)
The expression (7) can be generalized as the expression (8).
log Pa=Cxe2x88x92V/Sxe2x80x83xe2x80x83(8)
Herein, C is a proper constant of the sensor.
From the expression (8), in order to convert the output V of the pH electrode into the partial pressure Pa of the gas, the following can be understood: it is necessary that the two constants C and S are predetermined. To find C and S respectively corresponds to the zero point calibration and the sensitivity calibration. That is, in the same manner as in each kind of other sensor, also when the partial pressure of the gas is measured by using the severinghaus type gas sensor, it is inevitable to conduct the zero point calibration, and the sensitivity calibration. Further, when the temperature of the sensor changes at the time of the calibration and the measurement, it is also necessary to conduct the temperature compensation.
As described above, in the use of Severinghaus type gas sensor, or generally, of the chemical sensors, the two calibrations are inevitable, and the most troublesome matter for putting it to practical use.
In this connection, generally, the pH sensitivity of the pH electrode is theoretically given by following expression (9) of the Nernst"" equation.
S=2.303RT/Fxe2x80x83xe2x80x83(9)
Herein, R is a gas constant, T is absolute temperature, F is a Faraday constant, and when each constant is substituted in this equation, S at 25xc2x0 C. is 59 mV/pH. Then the equation (9) can be expressed as the expression (10).
S=59(273+t)/298 (mV/pH)xe2x80x83xe2x80x83(10)
Herein, t is the temperature (xc2x0 C.). When S is calculated from this expression, for example, it can be found that, when the temperature changes from 0xc2x0 C. to 40xc2x0 C., the pH sensitivity increases from 54 to 62 mV/pH. As described above, the sensitivity of the pH electrode is theoretically a function of only the temperature, and in many cases, the actual pH electrode has the sensitivity near the above theoretical value, and its time-dependent change is small. On the one hand, in many cases, there is a case where, for the zero point of the sensor, its time-dependent change can not be negligible.
Based on the above-mentioned trend, there are many cases to apply a method in which, for the sensitivity, the value measured in advance by the manufacturer of the sensor is used, and only the zero point calibration is conducted by the user, or the sensitivity calibration and the zero point calibration are conducted by the user with the sensitivity calibration being less often conducted than the zero point calibration. Specifically, as in the case of a PCO2 sensor for medical care with the comparatively narrower measuring range of PCO2, there is a case in which the necessary accuracy is secured only by the zero point calibration. In JP-A-11-070084, a package for a gas sensor is proposed for simply conducting such a zero point calibration. According to this, by delivering a package of laminated aluminum film in which both of the carbon dioxide of the predetermined partial pressure and the PCO2 sensor are accommodated as a product, the user can conduct the zero point calibration before the user opens the sensor package.
However, in the above gas package system, when the package is once opened, the zero point calibration can not be conducted. That is, although the zero point calibration can be conducted only once before the start of use, the zero point calibration can not be conducted thereafter. On the other side a sensor system by which the zero point calibration can be repeatedly conducted, is disclosed in JP-B-4-017050. This sensor system is the system in which a carrier solution (corresponds to the inner solution of the Severinghaus type gas sensor) is circulated in a gas exchange catheter located in the blood vessel, and the pH electrode is located at the downstream side of the gas exchange catheter, and the partial pressure of the gas is found from the pH of the carrier solution after the gas exchange. In this method, for example, by making the carrier solution flow at so high flow rate that the gas exchange in the gas exchange catheter may be negligible, the carrier solution in which the gas is hardly solved comes into contact with the pH electrode, thereby, the zero point calibration of the pH electrode can be conducted. That is, by only intermittently heightening the flow rate of the carrier solution, the zero point calibration of the pH electrode can be automatically conducted.
The above gas sensor system of carrier solution flow-through type is a system in which the automation of the zero point calibration can be easily carried out, however, it still has the following problems. (1) When base line drift of the pH electrode is considerable, it is necessary that the zero point calibration is frequently carried out. (2) The drift of the zero point due to the temperature change can not be corrected.
The object of the present invention is to provide a carrier solution flow-through type gas sensor system by which the automation of the zero point calibration can be easily carried out, and the baseline drift and the temperature drift of the pH electrode can be compensated.
Accordingly, the gas sensor of the present invention is constructed in such a manner that: a gas exchange section having a gas exchanger which is provided with an inlet and an outlet to circulate a carrier solution including at least ions conjugate to a gas to be measured, and separated from the outside by the gas permeable membrane; a forward path section which is connected to the inlet of the gas exchanger and guides the carrier solution to the gas exchanger; a return path section which is connected to the outlet of the gas exchanger and guides the carrier solution from the gas exchanger to the outside; a reference pH electrode arranged inside the forward path section; and a measuring pH electrode arranged in either of the inside of the gas exchanger or the inside of the return path section, are provided.
Further, in the gas sensor, when it is constructed such that the reference pH electrode is arranged at a position separated from the inlet, in the inside of the forward path section, and the measuring pH electrode is arranged at a position separated from the outlet, in the inside of the return path section, because the gas exchange section is separated from each pH electrode, the gas exchange section is easily taken off and made to be disposable. Further, because each pH electrode is separated from the gas exchange section as described above, the periphery of the reference pH electrode and the measuring pH electrode can be kept at constant temperature by a thermostatted block.
On the one hand, in the gas sensor, when the measuring pH electrode is constructed such that it is arranged inside of the return path section in the vicinity of the outlet, it becomes a sensor with quick response.
Further, in the gas sensor, when the measuring pH electrode is constructed such that it is arranged inside of the gas exchanger, further, it becomes a sensor with quicker response.
Further, in the gas sensor system of the present invention, it is constructed such that any one of the gas sensors and the potential difference detection means between the measuring pH electrode and the reference pH electrode are provided. Further, in the gas sensor system, it is constructed such that a carrier solution supply means for supplying the carrier solution from the other end of the forward path section, and a flow control means for controlling the flow of the carrier solution are provided. Further, in the gas sensor system, it is constructed such that a gas partial pressure detection means for detecting the partial pressure of the gas to be measured according to the output of the potential difference detection means is provided.
Further, in the gas sensor, by using two ISFET as the reference pH electrode and the measuring pH electrode, and a pseudo-reference electrode, detection of source potentials of the reference pH electrode and measuring pH electrode based on the pseudo-reference electrode is available.